This invention relates to facsimile transmitters of the type utilized in facsimile systems including a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to facsimile transmitters wherein a document is scanned so as to generate electrical information-bearing signals representing the dark-light variations in the document. These information-bearing signals may then be transmitted over the communications network or link to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a resonable facsimile of the original document.
In some instances, it is desirable to generate a digital signal representing the dark-light variations of the document. Such a digital signal, typically a binary signal, has a first level representing white or light areas of the document and a second level representing dark or black areas of the document. Such a binary signal is then utilized to control suitable modulation means such as a frequency shift keyed (FSK) signal generator which generates signals of two different frequencies representing the black-white variations in the document. Of course, a typical document, even one limited to alpha numeric matter, will be characterized by black areas, white areas and areas of intermediate darkness or gray areas. Where it is desirable to generate the abovedescribed binary signal, it becomes necessary to establish certain threshold levels in which areas lighter than a predetermined threshold of darkness will produce one level of the binary signal and areas darker than that threshold darkness will produce the other level of the binary signal.
Typically, the threshold level is fixed so that when the magnitude of an analog signal representing the dark-light variations exceeds the fixed threshold level, one level of the binary signal will be produced and when the magnitude of that signal is less than the fixed threshold, the other level of the binary signal will be produced.
Determining the level of the binary signal by comparison of the dark-light analog signal with a fixed threshold can create certain distortions in the information carried by a transmitted facsimile signal. For example, it is possible that a noticeable contrast in the document will be insufficient to permit the analog signal to cross a predetermined fixed threshold level. As a consequence, dark-light variations which should produce a black-white variation at a receiver produce only a single level of the binary signal. Moreover, it becomes virtually impossible to select a single threshold level for all documents. In one document where a great deal of contrast exists, a particular threshold level is appropriate. However, on a document which has less contrast (for example, the entire document may be quite dark including the background on which the information is written) a completely different threshold level is required. Furthermore, the use of such an all-important, single threshold level creates unreliability when that threshold level drifts for electrical or mechanical reasons with age and/or temperature.
These difficulties can be compounded where video peaking techniques are utilized to electrically enhance the fine detail of a document since video peaking can enhance noise as information. The term noise may include electrical noise as well as nonelectrical induced noise resulting from the typical uneveness of the copy paper structure and mechanical vibrations generated during scanning.